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POTASSIUM SULPHIDE                               gi

sulphuric acid ; when ferrous sulphate solution is poured carefully on to
the surface of the liquid, no brown coloration should be formed at the zone
of contact.

3. Quantitative Determination.—The solution is titrated with
ferrous ammonium sulphate or N/io-oxalic acid. To 10 c.c. of N/io-
oxalic acid, mixed with I c.c. of dilute sulphuric acid (1:4), the perman-
ganate solution (3-16 grams per litre) is run in until a persistent pink colora-
tion appears ; with the pure salt, 10 c.c. should be required.

POTASSIUM   PERSULPHATE

(See under Ammonium Persulphate)

POTASSIUM   SULPHATE

K2S04 = 174

The crude sulphate for fertilising purposes (see Potassium Salts, under
Fertilisers) and the pure sulphate, in large, colourless crystals soluble in
water, are on the market. The latter may contain small quantities of
chlorides, calcium, magnesium and sodium salts, and potassium hydrogen
sulphate. These impurities are detected as follows :

1.  Chlorides.—The solution, acidified with nitric acid, should give
no turbidity with silver nitrate.

2.  Metals.—The solution should not change with ammonium sulphide,
ammonia, ammonium oxalate or sodium phosphate.

3.  Sodium Salts.'—'These are recognised by the yellow coloration of
the flame.

4.  Bisulphate.—When bisulphate is present, the reaction is acid to
litmus paper.

POTASSIUM   SULPHIDE

K2S + 5H20 =200-2

The ordinary sulphide forms colourless, greenish or yellowish, deliquescent
crystals or fused, yellowish-red hygroscopic masses (K2S),and is not much
used. More common is a mixture of Potassium polysulphides with thio-
sulphate and sulphate, known as Liver of sulphur, which forms deliquescent,
greenish-yellow masses, reddish-brown inside, with a sulphurous odour,
largely soluble in water and partly so in alcohol (about 50%).

The analysis is carried out as with the corresponding sodium salt (q.v.) ;
i c.c. N/io-zinc sulphate = 0-05 gram K2S + 5lIaO = 0-02755 gram K2S.

For the analysis of liver of sulphur it is usually sufficient to verify the
external characters (the fracture should exhibit a reddish-brown liver colour)
and the reactions for polysulphides and potassium (with excess of hydro-
chloric acid, hydrogen sulphide should be evolved and sulphur deposited ;
if the liquid is then boiled and filtered and the filtrate evaporated to dryness,ater